The present invention is directed to a low cost process control system that will improve the reliability and performance of municipal and industrial wastewater treatment plants which are designed to remove phosphorus and nitrogen. The invention will serve as an enabling technology allowing engineers to better exploit the power of biotechnology to protect the environment and to significantly reduce the amount of energy consumed in removing nutrients from wastewater discharges.
Human activities can accelerate the rate at which nutrients enter ecosystems. Phosphorus (P) is often the limiting nutrient in cases of eutrophication in lakes and rivers subjected to runoff and/or point source pollution from wastewater treatment plants (Carpenter, Stephen R., “Phosphorus control is critical to mitigating eutrophication”, Proc Natl Acad Sci U.S.A.; Vol 105(32): 11039-11040 Aug. 12, 2008). At the same time, phosphate rock from which P fertilizers are produced is a non-renewable resource which is being rapidly depleted (Gilbert, N., “The disappearing nutrient”, Nature, Vol. 461, p. 719, October 2009). More effective process control systems will help to promote the maximum use of biological P (Bio-P) removal where chemical precipitation for P removal might otherwise be used. This is also important because P can be recovered as a by-product of Bio-P removal systems for beneficial use as fertilizer (Kresge, R., D. Barbeau, R. Bishop, K. Bowers, W. Leaf, and M. Murray, “Phosphorus Recycling for a Sustainable Future”, WEFTEC Proceedings, Orlando, Fla., October 2009), while P from chemical precipitates removed using metal salts cannot be effectively recovered for beneficial use.
Worldwide, biological treatment of municipal and industrial wastewater is one of the most important uses of biotechnology (Wagner, M.; Loy, A.; Nogueira, R.; Purkhold, U.; Lee, N.; Daims, H. “Microbial community compostition and function in wastewater treatment plants”, Antonie van Leeuwenhoek 2002, 81, 665-680). Although biological wastewater treatment has been used for more than a century, until recently, research on the microbiology of these treatment processes suffered from severe methodological limitations (Wagner, M.; Amann, R.; Lemmer, H.; Schleifer, K-H., “Probing activated sludge with oligonucleotides specific for proteobacteria: inadequacy of culture-dependent methods for describing microbial community structure”, Appl. Environ. Microbiol. 1993, 59, 1520-1525). With the introduction of several different molecular techniques in wastewater microbiology (Wagner, M.; Erhart, R.; Manz, W.; Amann, R.; Lemmer, H.; Wedi, D.; Schleifer, K-H. “Development of an rRNA-targeted oligonucleotide probe specific for the genus”, “Acinetobacter and its application for in situ monitoring in activated sludge”, Appl. Environ. Microbiol. 1994, 60, 792-800; Bond, P. L.; Hugenholtz, P.; Keller, J.; Blackall, L. L. “Bacterial community structures of phosphate-removing and nonphosphate-removing activated sludges from sequencing batch reactors, Appl. Environ. Microbiol. 1995, 61, 1910-1916; Schramm, A.; Larsen, L. H.; Revsbech, N. P.; Ramsing, N. B.; Amann, R.; Schleifer, K-H. “Structure and function of a nitrifying biofilm as determined by in situ hybridization and the use of microelectrodes”, Appl. Environ. Microbiol. 1996, 62, 4641-4647; Snaidr, J.; Amann, R.; Huber, I.; Ludwig, W.; Schleifer, K-H. “Phylogenetic analysis and in situ identification of bacteria in activated sludge”, Appl. Environ. Microbiol. 1997, 63, 2884-2896; Juretschko, S.; Timmermann, G.; Schmid, M.; Schleifer, K. H.; Pommerening-Roser, A.; Koops, H. P.; Wagner, M. “Combined molecular and conventional analyses of nitrifying bacterium diversity in activated sludge: Nitrosococcus mobilis and Nitrospira-like bacteria as dominant populations”, Appl. Environ. Microbiol. 1998, 64, 3042-3051; Lee, N.; Nielsen, P. H.; Andreasen, K. H.; Juretschko, S.; Nielsen, J. L.; Schleifer, K-H.; Wagner, M. “Combination of fluorescent in situ hybridization and micro-autoradiography—a new tool for structure-function analyses in microbial ecology”, Appl. Environ. Microbiol. 1999, 65, 1289-1297; Purkhold, U.; Pommering-Röser, A.; Juretschko, S.; Schmid, M. C.; Koops, H-P.; Wagner, M. “Phylogeny of all recognized species of ammonia oxidizers based on comparative 16S rRNA and amoA sequence analysis: implications for molecular diversity surveys”, Appl. Environ. Microbiol. 2000, 66, 5368-5382), it has become possible to determine the composition and dynamics of microbial communities in these systems and to identify the specific organisms which predominate in the different process reactor environments. One of the molecular detection methods commonly employed is the fluorescent in situ hybridization technique (FISH) targeting the 16S ribosomal RNA (rRNA) using oligonucleotide probes (short sequences of nucleic acids). More recently, DNA and RNA oligonucleotide microarrays (El Fantroussi, S.; Urakawa, H.; Bernhard, A. E.; Kelly, J. J.; Noble, P. A.; Smidt, H.; Yershov, G. M.; Stahl, D. A. “Direct profiling of environmental microbial populations by thermal dissociation analysis of native rRNAs hybridized to oligoneucleotide microarrays”, Appl. Environ. Microbiol. 2003, 69, 2377-2382; and Kelly, J. J.; Siripong, S.; McCormack, J.; Janus, L. R.; Urakawa, H.; El Fantroussi, S.; Noble, P. A.; Sappelsa, L.; Rittmann, B. E.; Stahl, D. A. “DNA microarray detection of nitrifying bacterial 16S rRNA in wastewater treatment plant samples”, Water Research 2005, 39, 3229-3238) and quantitative polymerase chain reaction (qPCR) targeting 16S rRNA and the polyphosphate kinase 1 gene from environmental microbial populations have been used. (He, Shaomei, Daniel L. Gall, and Katherine D. McMahon, ““Candidatus Accumulibacter” Population Structure in Enhanced Biological Phosphorus Removal Sludges as Revealed by Polyphosphate Kinase Genes”, Appl Environ Microbiol. Vol. 73(18): 5865-5874, 2007; McMahon, K. D., S. Yilmaz, S. He, D. L. Gall, D. Jenkins, and J. D. Keasling, “Polyphosphate kinase genes from full-scale activated sludge plants”, Appl. Microbiol. Biotechnol., Vol. 77 p. 167, August 2007).
Considerable efforts have been made over the years to develop improved on-line monitoring and control systems for wastewater treatment processes. However, until now the process control variables used in these systems have been based primarily on the measurement of physical and chemical parameters. As an example, dissolved oxygen (DO) sensors are widely employed to control aeration rates in activated sludge reactor basins. A control loop is used to maintain a desired DO set point by modulating the aeration input. Other chemical parameters which are often monitored in wastewater treatment systems include; ammonium (NH4+), nitrate (NO3−), nitrite (NO2−), and phosphate (PO4−3) ions. These parameters are principally used as indicators of overall treatment plant performance, and only infrequently and indirectly have they been used as on-line control process variables.
There is substantial and mounting evidence suggesting how the microbial community structure and their properties might be affected by the operation and control strategies which are employed in different wastewater treatment facilities (Yuan, Z. G.; Blackall, L. L. “Sludge population optimisation: a new dimension for the control of biological wastewater treatment systems”, Water Research 2002, 36, 482-490). However, the impact of these control actions on the microbial community in the system and on the specific organisms which are likely to predominant under a given set of operational conditions have not been explicitly taken into consideration until very recently (Yuan, Z.; Peng, Y.; Oehmen, A.; Huabing, L.; Fux, C.; Keller, J. “Sludge population optimisation in biological wastewater treatment systems through on-line process control: What can we achieve?” Proceedings of the 2nd IWA Conference on Instrumentation, Control and Automation. Busan, Korea, May 29-Jun. 2, 2005). Better integration of knowledge and tools between the disciplines of engineering and microbial biology is urgently needed to realize, fully, the great potential of environmental biotechnology (Daims, Holger, Michael W. Taylor and Michael Wagner “Wastewater treatment: a model system for microbial ecology”, TRENDS in Biotechnology Vol. 24 No. 11, 2006). Accomplishing this integration will require a systematic study on the effects of process design, operation and control on the microbial population dynamics in full-scale biological nutrient removal (BNR) treatment facilities using molecular biological methods. In this respect, wastewater treatment plants (WWTPs) create a significant opportunity for the needed cooperation between engineers and microbiologists in carrying out these studies. The present invention will make an important contribution to this effort which will also lead to important commercial applications, as well as broad societal benefits.
It is becoming increasingly common for regulatory agencies to impose limits on phosphorus as well as nitrogen on discharges from wastewater treatment plants (U.S. EPA 2009). The biological removal of both nutrients is more complex since the efficiency of both processes is dependent, in part, on the available organic carbon (C) substrate in the influent wastewater or from an external source, when necessary. Based on the knowledge which can be gained using the molecular detection tools described above to characterize and monitor the microbial communities in full-scaled wastewater treatment plants, the present invention will maximize biological phosphorus removal while at the same time achieving the maximum biological nitrogen removal which can be obtained with the available carbon in the influent wastewater. The system could be readily implemented in both new and existing treatment facilities.
The broader societal benefits will include: decreasing the environmental impact of humans on the planet and recovery of non-renewable natural resources, as noted above; significant educational opportunities; contributions to the scientific literature in this field; and the stimulation of technology transfer from molecular biologists to engineering professionals and wastewater treatment facility operating personnel.
In a conventional BNR wastewater treatment plant, nitrogen removal is achieved through nitrification and denitrification. Nitrification is the oxidation of ammonium to nitrate, catalyzed by bacteria, and is a key part of global nitrogen cycling (Prosser, J. I. “Autotrophic nitrification in bacteria”, Adv. Microb. Physiol. 1989, 30, 125-181). In the first step of nitrification, chemolithoautotrophic ammonia oxidizing bacteria (AOB) transform ammonium to nitrite. Nitrite is subsequently oxidized to nitrate by the nitrite-oxidizing bacteria (NOB) (Bock, E.; Koops, H-P.; Ahlers, B.; Harms, H. Oxidation of inorganic nitrogen compounds as energy source, p. 414-430. In Balows, A.; Truëper, H. G.; Dworkin, M.; Harder, W.; Schleifer, K-H.; The prokaryotes. 2nd ed.; Springer Verlag, New York, N.Y., 1992.). Data from 16S rRNA catalogs (Woese, C. R.; Weisburg, W. G.; Paster, B. J.; Hahn, C. M.; Tanner, R. S.; Krieg, N. R.; Koops, H. P.; Harms, H.; Stackebrandt, E. “The Phylogeny of the Purple Bacteria: The Beta Subdivision”, Systematic and Applied Microbiology 1984, 5, 327-336; Woese, C. R.; Weisburg, W. G.; Hahn, C. M.; Paster, B. J.; Zablen, L. B.; Lewis, B. J.; Macke, T. J.; Ludwig, W.; Stackebrandt, E. “The Phylogeny of the Purple Bacteria: The Gamma Subdivision”, Systematic Applied Microbiology 1985, 6, 25-33) first demonstrated that there are two phylogenetically distinct groups of autotrophic AOBs. One of these contained Nitrosococcus oceanus, and was within the gamma-subdivision of the Proteobacteria. The AOB in the beta-subdivision formed two deep branches; Nitrosomonas and Nitrosospira (Head, I. M.; Hiorns, W. D.; Embley, T. M.; McCarthy, A. J.; Saunders, R. “The Phylogeny of Autotrophic Ammonia-Oxidizing Bacteria as Determined by Analysis of 16S Ribosomal RNA Gene Sequences”, Journal of General Microbiology 1993, 139, 1147-1153). All isolated chemolithoautotrophic, nitriteoxidizing bacteria belong to one of four different genera; Nitrobacter (alpha subclass of Proteobacteria), Nitrococcus (gamma subclass of Proteobacteria), Nitrospina (delta sub-class of Proteobacteria), and Nitrospira (phylum Nitrospira) (Bock, E.; Koops, H-P. The genus Nitrobacter and related genera. p. 2302-2309. In Balows, A.; Truëper, H. G.; Dworkin, M.; Harder, W.; Schleifer, K.-H. The prokaryotes, 2nd ed.; Springer Verlag, New York, N.Y., 1992).
It had long been assumed that Nitrosomonas spp., were responsible for oxidizing ammonium to nitrite (Equation 1) and that Nitrobacter spp., were responsible for oxidizing nitrite to nitrate (Equation 2) (Randall, C. W.; Barnard, J. L.; Stensel, H. D. Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal. Vol. 5, Technomic Publishing Co., Lancaster, Pa., 1992; Bever, J.; Stein, A.; Teichmann, H. (ed.). Weitergehende Abwasserreinigung. R. Oldenbourg Verlag, Munich, Germany, 1995; Henze, M.; Harremoës, P.; Jansen, J.; Arvin, E.; Wastewater treatment. 2nd ed.; Springer-Verlag, Berlin, Germany, 1997):2NH4++3O2→2NO2−+4H++2H2O  Equation 12NO2−+O2→2NO3−  Equation 2
However, in contrast to this “textbook” knowledge, Nitrospira-like bacteria, not Nitrobacter spp., have been observed to be the dominant nitrite oxidizers, both in most full-scale wastewater treatment plants and in laboratory scale reactors (Juretschko, S.; Timmerman, G.; Schmid, M.; Schleifer, K. H.; Pommerening-Roser, A.; Koops, H. P.; Wagner, M. “Combined molecular and conventional analysis of nitrifying bacterium diversity in activated sludge: Nitrosococcus mobilis and Nitrospra-like bacteria as dominate populations”, Appl. Environ. Microbiol. 1998, 64, 3042-3051; Okabe, S.; Satoh, H.; Watanabe, Y. “In situ analysis of nitrifying biofilms as determined by in situ hybridization and the use of microelectrodes”, Appl. Environ. Microbiol. 1999, 65, 3182-3191; Schramm, A.; de Beer, D.; Wagner, M.; Amann R. “Identification and activities in situ of Nitrosospira and Nitrospira spp. as dominant populations in a nitrifying fluidized bed reactor”, Appl. Environ. Microbiol. 1998, 64, 3480-3485; Wagner, M.; Rath, G.; Koops, H-P.; Flood, J.; Amann, R. “In situ analysis of nitrifying bacteria in sewage treatment plants”, Water Sci. Technol. 1996, 34, 237-244). Based on fluorescence in situ hybridization combined with microelectrode measurements, it has been suggested that Nitrospira-like nitrite oxidizers represent K strategists adapted to low nitrite and oxygen concentrations, while Nitrobacter spp., is an r strategist which thrives if nitrite and oxygen are present in higher concentrations (Schramm, A.; de Beer, D.; van den Heuvel, J. C.; Ottengraf, S.; Amann, R. “Microscale distribution of populations and activities of Nitrosospira and Nitrospira spp. along a macroscale gradient in a nitrifying bioreactor: quantification by in situ hybridization and the use of microsensors”, Appl. Environ. Microbiol. 1999, 65, 3690-3696). Likewise, it has been observed in some wastewater treatment plants that the Nitrosospira genus was the dominant AOB rather than Nitrosomonas (Coskuner, G.; Curtis, T. P. “In situ characterization of nitrifiers in an activated sludge plant: detection of Nitrobacter Spp”, J. Applied Microbiology 2002, 93, 431-437).
Although organic carbon is not required for nitrifying bacteria to complete nitrification, sufficient organic carbon in the form of readily biodegradable chemical oxygen demand (rbCOD) is necessary for denitrifying bacteria to carry out denitrification. To achieve enhanced biological phosphorus removal (EBPR) as depicted in FIG. 1, it is necessary to provide the phosphorus accumulating organisms (PAO) with adequate supply of rbCOD in form of volatile fatty acids (VFA) and appropriate anaerobic conditions free of nitrates. Thus, there is a competition for the available rbCOD between PAOs and denitrifying bacteria. When influent wastewater is weak or dilute, external carbon needs to be added. In order to solve the problem of competition for the limited organic substrate, denitrifying phosphorus accumulating organisms (DNPAO) have received considerable attention (Kuba, T., van Loosdrecht, M. C. M, & Heijnen, J. J. “Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying dephosphatation and nitrification in a two-sludge system”, Water Res. 30 (7) 1702-1710, 1996; Spagni, A., Stante, L. & Bortone G., “Selection of denitrifying phosphorus accumulating organisms in activated sludge”, Env. Technol. 22 (12), 1429-1437, 2001; Ahn, J., Daidou, T., Tsuneda, S. & Hirata, A. “Selection and dominance mechanisms of denitrifying phosphate-accumulating organisms in biological phosphate removal”, Biotechnol. Letters. 23, 2005-2008, 2001; Shoji, T., Satoh, H. & Mino, T. “Quantitative estimation of the role of denitrifying phosphate accumulating organisms in nutrient removal”, Water Sci. Technol. 47 (11) 23-29, 2003).
DNPAO are distinguished from aerobic PAO by their unique metabolic characteristic (Murnleitner, E., Kuba, T., van Loosdrecht, M. C. M, & Heijnen, J. J. “An integrated metabolic model for the aerobic and denitrifying biological phosphorus removal”, Biotechnol. Bioeng. 54 (5) 434-450, 1997). The mechanism of anaerobic phosphate release of DNPAO is the same as that of aerobic PAO. As depicted in FIG. 1, external organic substrate is taken up and converted to polyhydroxyalkanoate as a cell energy source (The Figure shows PHB, or polyhydroxybutyrate as the cellular storage polymer). However, the manner of phosphate uptake is different between the two types of PAOs. Aerobic PAOs can only use oxygen as an electron acceptor for cell respiration which promotes phosphorus removal; whereas DNPAO can use nitrite or nitrate as an electron acceptor instead of oxygen (Meinhold, J., Arnold, E. and Isaacs, S., “Effect of nitrite on anoxic phosphate uptake in biological phosphorus removal activated sludge”, Water Res. 33, 1871-1883, 1999). In this way, DNPAO can combine phosphorus removal and denitrification into one process using the same amount of organic carbon substrate. In addition, less aeration is needed which translates into lower energy requirement. Kuba et al. (“Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying dephosphatation and nitrification in a two-sludge system”, Water Res. 30(7) 1702-1710, 1996) showed that DNPAO can reduce sludge generation by 30%, due to their low cell yield. Thus, the advantage of selecting for DNPAOs over PAOs by means of reactor configuration and/or control strategies is very significant. Therefore, selection and enrichment of DNPAO is a key factor in optimizing EBPR and biological nitrogen removal and this is one of the objectives of the present invention.
Although denitrifying phosphate uptake was reported earlier by Gerber (Gerber, A., de Villiers, H., Mostert, E. S. & van Riet, C. J. “The phenomenon of simultaneous phosphate uptake and release, and its importance in biological nutrient removal. In: Biological Phosphate Removal from Wastewater”, Advances in Water Pollution Control No. 2; R. Ramadori (ed.). Pergamon Press, Oxford, pp. 123-134, 1987) and Comeau, Y., Oldham, W. K. & Hall, K. J. “Dynamicas of carbon reserves in biological dephosphatation of wastewater. In: Biological Phosphate Removal from Wastewater”, Advances in Water Pollution Control No. 2. R. Ramadori (ed.). Pergamon Press, Oxford, pp. 39-55, 1987, DNPAO did not receive a great deal of attention until Kerrn-Jespersen, J. P. & Henze, M., “Biological phosphorus uptake under anoxic and aerobic conditions”, Water Res. 27, 617-624, 1993, Henze, M.; Harremoes, P.; Jansen J.; Arvin, E, Wastewater Treatment 2nd ed., Springer-Verlag, Berlin, Germany (1997) distinguished them from aerobic PAO.
Later research focused more on DNPAO metabolic characteristics and biochemical mechanisms (Kuba, T., Smolders, G., van Loosdrecht, M. C. M, & Heijnen, J. J., “Biological phosphorus removal from wastewater by anaerobic-anoxic sequencing batch reactor”, Water Sci. Tech. 27 (5-6), 241-252, 1993; Sorm, R., Wanner, J. Saltarelli, R., Bortone, G. & Tilche, A., “Verification of anoxic phosphate uptake as the main biochemical mechanism of the “Dephanox” process“, Water Sci. Tech. 35 (10), 87-94, 1997. Chung, J. Kim, Y., Lee, D. Shim, H. & Kim, J., “Characteristics of denitrifying phosphate accumulating organisms in anaerobic-intermittently aerobic process”, Environ. Eng. Sci. 23 (6) 981-993, 2006). However, little attention was paid to its selection and enrichment. More recently, Yuan and Oleszkiewicz (“Selection and Enrichment of Denitrifying Phosphorus Accumulating Organisms in Activated Sludge”, WEFTEC Proceedings, Orlando, Fla., October 2009) observed a community shift favoring DNPAOs over PAOs in laboratory scale sequencing batch reactors (SBRs) by extending the anoxic period and increasing nitrate dosage.
The basic engineering principles for the design of P and N removal treatment facilities have been well established and have been implemented in a number of configurations including the UCT process, the Bardenpho process, A2O, and others. The UCT Process flow configuration is shown in FIG. 2. All of these processes include an anaerobic zone, followed by one or more anoxic zones and one or more aerobic zones. The only systems that would not include an anoxic zone would be those that are not required to and do not nitrify. For all others, it is imperative that any return activated sludge or mixed liquor be denitrified before being returned to the anaerobic zone.
When nitrate or oxygen is discharged to the anaerobic zone, two things may happen, both undesirable:                They will prevent fermentation of rbCOD to acetic and propionic acid; and        Nitrates or DO could serve as electron acceptors for PAOs and other organisms that will metabolize the VFA and so deprive the PAOs of the substance that they need to store for growth and phosphorus removal.        
Therefore, the UCT and other similar processes utilize internal recycle streams such that minimal nitrate or DO is returned to the anaerobic zone. These pumped recycle stream flow rates must be carefully regulated to assure that the desired process objectives are achieved. These types of BNR facilities are more complex than conventional activated sludge plants in terms of operation and control and are more expensive to construct, operate, and maintain. Because of the added cost and complexity of multi-stage BNR systems, many WWTPs, particularly smaller plants less than 5 MGD, have up to now elected to utilize chemical precipitation instead when needed to meet permit requirements for phosphorus removal.
In recent years there has been a great deal of interest focused on treatment plant designs and control strategies which achieve simultaneous nitrification and denitrification (SNdN) in the same reactor instead of separate anoxic/aerobic reactors with internal recycle pumping. The typical approach to encourage SNdN is to limit the DO concentration to very low levels. Advantages which can be attributed to SNdN as compared to more conventional nitrogen removal processes include:                1. Reduced aeration demand for nitrification (lower energy consumption).        2. Internal recycle not required (lower capital costs, less energy, and simpler operation).        3. Reduced energy consumption due to improved oxygen transfer driving force at low DO set points.        4. Reduced carbon source requirements to drive denitrification of NO2− instead of NO3− if the system can be operated to inhibit the growth of NOBs. The effective inhibition of NOB has been referred to as the “nitrite shunt” (O'Neill, M.; Huren, N.J. “Achieving Simultaneous Nitrification and Denitrification of Wastewater at Reduced Costs”, Water Sci. Technol. 1995, 32 (9-10), 303-312.        
If an SNdN reactor were to follow an anaerobic zone as shown in FIG. 3, it might be possible to achieve P removal as well as N removal while realizing the benefits of SNdN listed above. However, it has been shown, that systems operated at very low DO levels as necessary for SNdN have poorer P removal performance than those operated at higher DO levels (Drury, Douglas D., William Shepherd and B. Narayanan, “Phophorus—How Low Can You Go?”, WEFTEC Proceedings, 2005).
Another alternative for achieving nitrification and denitrification in a single reactor is to cycle the basin between oxic and anoxic conditions. It is possible to detect the depletion of nitrate in a cyclic aerobic/anoxic process by following the rate of change in the oxidation reduction potential in the mixed liquor (Stensel, H. D.; Coleman, T. E.; Fleischmann, D.; Denham, W. B. “Innovative Process Used to Upgrade Oxidation Ditch for Nitrogen Removal and SVI Control”, Proceedings of the Water Environment Federation Conference, Orlando Fla., October, 1995; Coleman, T. E.; Stensel, H. D.; Fleischmann, D.; Denham, W. B. “Automated ORP control for aerobic/anoxic cycling in oxidation ditch treatment”, Proceedings of the Amer. Soc. Of Civil Engr. National Conference on Environ. Engr., Boulder Colo., Jul. 11, 1994.)). If aeration is turned off at a selected time, the ORP will fall rapidly at first and then more gradually until a second inflection point is reached at a much lower ORP value. This second inflection point indicates the depletion of nitrate. FIG. 4 is a typical graph of the ORP vs. time when this aerobic/anoxic cyclic process is implemented. An advantage of the oxic/anoxic cycling approach is that is would avoid long term low DO conditions which lead to reduced P uptake. It could also provide selective pressure favoring DNPAOs.